In a sliding caliper disc brake, the brake pad on one side moves with respect to the caliper, towards the disc, when the brake is actuated and with pad wear. The brake pad on the other side of the disc is fixed relative to the caliper. To provide a clamping force to both sides of the disc, the whole caliper moves axially relative to the brake disc, when the brake is actuated, to bring the brake pad which is fixed to the caliper into contact with the brake disc. The whole caliper also moves axially relative to the brake disc to allow for wear of the pad and disc.
To allow this to happen, the caliper is typically mounted onto a brake carrier by means of two guide pins with bushes. The guide pins are bolted to the brake carrier, and run in bushes within bores in the brake caliper. The guide pin arrangement should be sealed, ideally at both ends, to protect against water and dirt ingress from the road. However, due to the very limited space available in some vehicles, it is often not possible to provide good sealing whilst allowing for caliper movement to adjust for pad and disc wear.
In particular, a common design for a sliding caliper brake includes a long guide pin and a short guide pin. The long guide pin slides entirely within a bore in the caliper, which is long enough to surround the guide pin over the full range of movement of the caliper due to pad and disc wear. One end of the bore can therefore be closed by a protective cap, substantially preventing ingress of dirt. However, the shorter guide pin generally slides within a short through-bore in the caliper, where a shorter guide pin is required to prevent fouling with other vehicle components. The shorter guide pin passes all the way through the caliper and protrudes from the other side in the new pad condition, but when the pads wear the caliper slides away from the carrier, so any protective cap may intrude into other vehicle components in the worn pad condition. In this situation it is impossible to install a protective cap over the entire bore.
One way in which partially sealing the short guide pin has been attempted is to provide a wiper seal. This does not stop dirt from collecting on the exposed surface of the guide pin, thereby initiating corrosion of the guide pin, but hopefully limits the amount of dirt which can ingress between the outer surface of the guide pin and the inner surface of the bore in the caliper. However, this is only of limited effectiveness, and the presence of the wiper seal increases the frictional resistance to the guide pin sliding within the bore. Also, to achieve an acceptable service life, the guide pin has to be made from expensive stainless steel to inhibit corrosion.
Another partially-effective way of sealing the short guide pin is to provide a rolling boot which is fixed to the interior of the caliper bore at one end, and extends away from the bore. The rolling boot includes a sealing bead at the other end which is fixed to the extreme end of the guide pin. This is done by seating the sealing bead around the end of the guide pin, and then pushing a metal retaining ring into the counterbore of the guide pin, which is provided for bolting the guide pin onto the brake carrier. However, the metal retaining ring is prone to corrosion, and in extremes of movement the sealing bead can come loose from between the retaining ring and the end of the guide pin. The sealing system relies on a frictional press fit between the outer surface of the retaining ring and the inner surface of the counterbore to prevent the retaining ring from coming loose, and therefore the surface area of contact is ideally maximised. The retaining ring is therefore designed to be long enough to push up against the guide pin bolt, but the sealing performance can then be affected by manufacturing tolerances in the bolt. Very well controlled manufacturing tolerances in the bolt, the retaining ring and the bore are required for the press fit to work well.
Sealing the guide pin assembly at the outboard end (i.e. the end nearest the brake carrier) can also present difficulties. This is essentially because the clearance between the brake carrier and the caliper can be very limited. Where there is space, a rubber boot can be provided at the outboard end between the caliper and the guide pin. This forms an effective seal and is often provided on the longer of the two guide pins. However, for the short guide pin there is often not enough space to provide the known rubber boot arrangement. This end of the short guide pin is therefore often left unsealed which reduces reliability and service life.
It is an object of the invention to provide effective sealing arrangements for sliding caliper guide pins, where there is limited space.